A technique is desired, which can attain an integrated circuit of optical elements, such as an integrated circuit of transistors. Under present circumstances, an optical circuit is produced such that individual elements, such as optical fibers serving as waveguides, discrete optical switches, wavelength filters and 3 dB couplers, are connected. However, if this optical circuit can be integrated in a small chip, the volume, electric power consumption and manufacturing cost of the circuit can be dramatically reduced.
Until now, many techniques have been developed for attaining the optical integrated circuit. Among those techniques, a technique of photonic crystal is remarked as the technique that can advance the higher function, smaller size and smaller electric power consumption of an optical device on a substrate at the digits between 100 and 100,000,000 times.
In a broad sense, the photonic crystal is the generic name of a structure in which a refractive index is periodically changed. The photonic crystal is typically used for electromagnetic wave. This name was given because this was originally devised for optical applications, and the periodic structure was analogous to crystals. The photonic crystal has various peculiar optical features because of the periodicity of its refractive index. The most representative feature lies in the possession of photonic band gaps (PBGs). When the periodic refractive index change of a photonic crystal is very large, light in a particular frequency band (or a wavelength band) cannot propagate through the photonic crystal. The diagram in which a longitudinal axis indicates the frequency of light, a lateral axis indicates the wave number of light, and the relation between the frequency and wave number of light propagating through the photonic crystal is plotted is referred to as a dispersion relation diagram or a photonic band diagram. In a photonic band diagram, frequency bands (or wavelength bands) in which such plots continuously exist and distribute as curves are referred to as bands. The gaps between those bands are referred to as photonic band gaps (PBGs) since the lights with the frequencies in the gaps cannot propagate through the photonic crystal.
When a micro defect that disturbs the periodicity of the refractive index distribution of a photonic crystal exists in the photonic crystal, lights having frequencies in PBGs are confined in the micro defect. In that case, since only lights having frequencies corresponding to the size of the defect are confined, the photonic crystal acts as a resonator of the lights and can be used as a frequency (wavelength) filter. When a line defect in which the micro defects are continuously formed in a line exists in the photonic crystal, even the lights having the frequencies in the PBGs can propagate along the line defect while confined in the line of the line defect. This indicates that the line defect of the photonic crystal carries out the role as a waveguide, and this is referred to as a line-defect waveguide. If a filter and a waveguide can be formed, optical modulators, optical switches and the like can be configured from only the waveguide or from a combination of the waveguide and the filter. In this way, when there is the photonic crystal, all of the main optical functional elements can be formed therein and connected, thereby enabling the configuration of optical circuits. Thus, the photonic crystal is expected as a platform of optical integrated circuits.
As a typical feature, the photonic crystal is useful as a platform of optical integrated circuits. From the viewpoint of manufacturing, the periodicity of the photonic crystal is desired to be two-dimensional. When the effect of the PBG is needed in the three directions of x, y and z which are perpendicular to one another, the photonic crystal must be three-dimensional. However, since the three-dimensional structure is complex, the manufacturing cost becomes expensive. So, a two-dimensional photonic crystal having a definite thickness is used, which has the two-dimensional periodicity inside a substrate plane and does not have periodicity in a thickness direction. In that case, the optical confinement in the thickness direction in a line-defect waveguide or a point-defect resonator is not caused by the effect of PBG, but caused by the mechanism of the total internal reflection due to the refractive index difference. The characteristics of the two-dimensional photonic crystal of the definite thickness do not perfectly coincide with those of the two-dimensional photonic crystal having an indefinite thickness. However, the optical characteristics substantially coincides with those of the two-dimensional photonic crystal having the indefinite thickness, if the refractive index distribution in the thickness direction of the two-dimensional photonic crystal having the definite thickness exhibits the reflection symmetry in the region through which light propagates. Predicting operation of a device composed of the two-dimensional photonic crystal having the indefinite thickness is very easy as compared with predicting operation of a device having a definite thickness. Thus, if the two-dimensional photonic crystal in which the refractive index distribution exhibits the reflection symmetry can be used, designing a device composed of the crystal will be easy. As specific structures which have been attained as two-dimensional photonic crystals having definite thicknesses, there are air-hole photonic crystal and pillar photonic crystal. Among them, in particular, line-defect waveguides of the latter crystal has useful propagation properties.
In a typical structure of the pillar photonic crystal of the definite thickness, cylindrical pillars that have a definite height and are made of high-dielectric-constant materials are arranged in a shape of square-lattice. In the square-lattice-of-cylindrical-pillar photonic crystal having the definite thickness as mentioned above, it is possible to form a waveguide in which light propagates along the line-defect-cylindrical pillars by making a cross section of a cylindrical pillar (referred to as a line-defect-cylindrical pillar) arranged in a certain line in the crystal, smaller than a cross section of a cylindrical pillar arranged around the certain line. In this waveguide, the line of the line-defect-cylindrical pillars corresponds to a core in a total-internal-reflection-confinement-type waveguide such as optical fibers and the like, and the cylindrical-pillar lattices distributed on both sides of the line of the line-defect-cylindrical pillars correspond to a cladding layer.
The feature of the line-defect waveguide lies in that a group velocity of guided light is small. Therefore, the line-defect waveguide can be used as an optical delay device. Also, because of the small group velocity, interaction time for the guided light and the crystal material becomes long. As a result, interaction effect becomes great even in a short waveguide. Thus, the line-defect waveguide can be also used as a waveguide in which a non-linear effect and the like can be efficiently used. On the other hand, depending on an application, there is a case where such characteristics are not appropriate. For example, when the line-defect waveguide is used as an optical interconnection to merely connect an optical delay device and a point defect resonator filter in an optical circuit, it is desired that the group velocity of the guided light is large. This is because the processing speed of the circuit can be high. Therefore, it is important to choose appropriate waveguides for different purposes of applications in the optical circuit. Hence, a technique is important, which can optically couple at a high efficiency the line-defect waveguide of the square-lattice-of-cylindrical-pillar photonic crystal that operate at the low group velocity and the wire waveguide that operates at the relatively high group velocity.
However, when both of the waveguides are simply butt-jointed to each other, highly efficient optical coupling cannot be obtained. This is because distribution of electromagnetic field intensity distributions and distributions of impedance (a ratio between an electric field and a magnetic field) of guided lights are greatly different between the guided lights in the two connected waveguides. Therefore, optical coupling structures for adiabatically connecting both of the waveguides, namely, the structure for gradually changing the waveguide structure from the structure of the line-defect waveguide to the structure of the wire waveguide have been devised. By adiabatically changing the waveguide structure, it is possible to gradually change the electromagnetic field distribution of the guided lights in a wide region and match the electromagnetic field impedances of the guided lights in the wide region in all of the portions of the optical coupling structure. Incidentally, the terms of “coupling” and “optical coupling” in this specification mean that optical electromagnetic field energy is transmitted from guided light of one waveguide to guided light of another waveguide. Also, the terms of “optical coupling structure” and “coupling structure” mean the structure for the optical coupling. On the other hand, the term “connection” is used to mean that the waveguides are merely structurally connected to each other.
An example of the optical coupling structure of a conventional waveguide is described below. FIG. 1 is a schematic diagram showing an example of the optical coupling structure of the waveguides disclosed in a document (Steven G. Johnson et al., Physical Review E, vol. 66, p. 066608). This drawing shows the cross section in the direction parallel to the substrate including the waveguide of light. An optical coupling structure 8 in FIG. 1 employs a configuration in which two taper portions (first taper portions 2) whose structures are adiabatically changed are combined. In this document, those two taper portions are shown in FIG. 8 and FIG. 9(b) of the document, respectively.
Incidentally, in this document, as a pillar of the pillar photonic crystal, a quadratic pillar 5 is used instead of the cylindrical pillar. The first taper portion 2 shown in FIG. 1 has the structure in which in a square-lattice-of-quadratic-pillar photonic crystal, lattices of quadratic pillars located, on both sides of a line of line-defect pillars (defect quadratic pillars 6) of a line-defect waveguide (line-defect waveguide portion 1) are gradually away from the line of the defect quadratic pillars 6. Also, a second taper portion 3 shown in FIG. 1 has a structure in which in the line of only the defect quadratic pillars that remains in the end of the first taper portion 2, a quadratic pillar interval is gradually narrowed and finally changed to a shape of a thin wire. FIG. 1 shows a structure in which the interval between gravity centers of the quadratic pillars is gradually narrowed, in order to make the quadratic pillar interval narrow in the second taper portion 3. On the other hand, FIG. 4 in this document shows a structure in which, instead of changing the gravity center interval between the quadratic pillars, a length in the waveguide direction, of the quadratic pillar is made gradually long and finally changed to the shape of the thin wire.
As a related technique, Japanese Laid-Open Patent Application (JP-P 2005-172933A) discloses a manufacturing method of a two-dimensional photonic crystal with a wire waveguide. This manufacturing method is a method of manufacturing the two-dimensional photonic crystal with the wire waveguide, which is composed of a two-dimensional photonic crystal having a waveguide and a wire waveguide connected to the waveguide, from a plate material in which a slab layer and a clad layer are laminated. The manufacturing method is characterized by having: (a) a vacancy forming step of forming an etching agent introducing vacancy in the slab layer; (b) an air bridge cavity forming step of introducing an etching agent through the etching agent introducing vacancy, and consequently etching a clad layer around the etching agent introducing vacancy to form a cavity in the clad layer; and (c) a step of forming a two-dimensional photonic crystal with a wire waveguide, in which vacancies are cyclically formed in the slab layer facing the cavity, then an in-crystal waveguide is formed from an outer edge of a region where the vacancies are formed to an inner side thereof to form the two-dimensional photonic crystal, and the slab layer is left by a predetermined width on the extension portion of the in-crystal waveguide from an outer edge of a region where the vacancies are formed to an outer side thereof, then the slab layer around it is removed to form the wire waveguide.
Also, Japanese Laid-Open Patent Application (JP-P 2003-315572A) discloses an optical waveguide and an optical element using it. This optical waveguide has a core portion and a clad portion having a photonic crystal member and may be configured such that a structure of the photonic crystal member is changed, and an effective refractive index of the clad portion is spatially changed, and a mode field diameter which is an electronic field magnitude distribution inside a plane vertical to an advancement direction of an optical waveguide mode is spatially changed. This may be configured such that a basic waveguide mode exists as the optical waveguide mode, and the mode field diameter of the basic waveguide mode is spatially changed.
Also, Japanese Laid-Open Patent Application (JP-P 2003-270458A) discloses a photonic crystal member and a photonic crystal waveguide. This photonic crystal waveguide is characterized in that in the photonic crystal member to propagate an electromagnetic wave, the photonic crystal waveguide is provided with a photonic crystal for a reflection protection of the electromagnetic wave. In the photonic crystal waveguide which propagates and guides the electromagnetic wave by using the photonic crystal member, this photonic crystal waveguide may be configured such that a region of a photonic crystal whose structure differs from the foregoing crystal member is provided on an input side or an output side of the electromagnetic wave in the crystal member, and an output magnitude from the crystal member is made large as compared with the case without the foregoing region.
The conventional coupling structure between the square-lattice-of-pillar photonic crystal line-defect waveguide and the wire waveguide has several problems. The first problem lies in the fact that it is difficult to obtain an optical coupling efficiency as designed in the conventional coupling structure of the waveguides, and thereby transmission characteristics and productivity are poor. This problem is caused by the fact that the portion difficult to be processed as designed exists in the coupling structure. This is because in the second taper portion 3 in FIG. 1, when the pillar interval becomes gradually narrow, the pillar interval becomes unlimitedly close to zero, immediately before reaching of the thin wire of the wire waveguide and exceeds a limit of a processing accuracy. In the second taper portion 3, to obtain a wide band and a high transmittance as much as possible, a rate at which the pillar intervals decrease is required to be as small as possible. However, as a result, the portion where the pillar interval is extremely narrow distributes over a long distance, and that portion cannot be well processed. Thus, the transmission efficiency will be worse on the contrary. The second problem lies in the fact that the length of the conventional coupling structure in the propagation direction of light will be long. In highly integrated optical circuits, it is desired that optical coupling structures in them are as small as possible. Hence, the long coupling structure can be a problem. This problem is caused by the fact that the coupling structure of the waveguides is configured by not one taper portion but a combination of two taper portions. In short, when periodicity of the crystal lattice portion of the pillar is changed in the coupling structure, this is not well operated, which disables the attainment of one taper structure that carries out functions of the first taper portion 2 and the second taper portion 3 at the same time.